A method is known for uniaxial single-action compacting of powder materials in closed molds comprising a matrix with a passive shaping surface that does not transfer the pressing force to the powder being compacted, and punches with shaping surfaces that are capable of transferring the pressing force to the powder being compacted (see e.g. Fedorchenko I. M., Frantzevich I. N., Radomyslensky I. D. et al., Powder Metallurgy. Materials, Processing, Properties, Fields of Application, Kiev, Naukova dumka, 1985). The compacting is accomplished by transferring the pressing force to the powder through the active shaping surface of one of the punches. The prior method permits fabrication of articles of Complexity Groups I and II having a shape factor value close to unity.
When articles of complexity group III are manufactured by this method, in order to reduce the value of compacting density differential along the article height by one half as compared to a single-action compacting scheme, a double-action uniaxial compacting is implemented. To provide the same conditions for articles of Complexity Groups IV-VII having different changes in height, the use is made of composite punches with independently moving components, and specialized multi-pass presses with synchronized and controlled travel of their components.
The basic problem with the prior art method is that in all compacting routes the average density distribution of the powder article through its cross-section normal to the pressing axis, along the article height and throughout the volume has an explicitly nonuniform character with the layers of the same density being bent in the direction of the pressing punch movement (Shtern M. B., Serdyuk G. G., Maximenko L. A., Trukhan Y. V., Shulyakov Y. M. Phenomenological Theories of Powder Compacting, Kiev, Naukova dumka, 1982).
In the conditions of large-scale production of powder articles of Complexity Groups I, II, III, multi-form molds are used, this many times enhancing labor intensity as the number of shaping components (punches and cavities of the multi-form matrix) corresponds to the number of articles compacted at once. When compacting articles of irregular shape with a developed surface, split dies are used in order to eliminate destructive impact of elastic aftereffect (Klyachko L. I., Umansky A. M., Bobrov V. N. Equipment and Accessories for Forming Powder Materials, Moscow, Metallurgy, 1986), this increases the number of die components and complicates the process of die fabrication and operation, but the problems of obtaining an acceptable uniform density of pressed articles along the height still remain.
Most closely related to the present invention is a method for compacting sleeves with counter movement of a matrix and an insertion rod, comprising the steps of: placing a powder material in a shaping cavity of a mold, the shaping cavity being defined by active and passive shaping surfaces of one-piece or composite shaping members of the mold; mutually moving the shaping members of the mold along the pressing axis, with the pressing force transferred from the shaping members of the mold to the powder material through the active shaping surfaces; and forming surfaces of the powder article, parallel to the pressing axis, by passive shaping surfaces of the one-piece or composite shaping members (see e.g. Popilsky R. Y., Pivinsky Y. E. Compacting Powder Ceramic Masses, Moscow, Metallurgy, 1983).
In the prior method, the passive shaping surfaces, located on the matrix and the insertion rod, form the external and internal side surfaces of the article, while the active shaping surfaces, located on the punches, form end faces of the article. Being rigidly connected, the matrix and one pressing punch and the rod and the other pressing punch accomplish mutual counter movement, and the pressing force is transferred through the active shaping surfaces. Such an arrangement permits fabrication of sleeve-shaped articles with more uniform density distribution along the height of the article.
Under the same friction conditions on both of the passive shaping surfaces defining the internal and external side surfaces of the article when it is compacted with counter movement of the matrix and the insertion rod, the average density differential in the section normal to the pressing axis along the height cannot be zero as it is determined by the difference in the areas of the opposite side surfaces, external and internal.
Differentials of the average axial pressure, ΔP, in the cross section and the average powder body density, Δρ, along its height, when a sleeve is compacted with counter movement of the matrix and the rod, depend on factors of wall friction f and lateral pressure ξ, height h of the article and values of external radius r1 and internal radius r2 of the sleeve being compacted:                               Δ          ⁢                                           ⁢          P                =                              2            ·            f            ·                          P              b                                ⁢                      h                                          r                2                            +                              r                1                                                                        (        1        )                                          Δ          ⁢                                           ⁢          P                =                              2            ·            f            ·            ξ                    ⁢                                           ⁢                      h                                          r                2                            +                              r                1                                                                        (        2        )            
where Pb is the average lateral pressure along the height of the article.
The method of compacting sleeves with counter movement of the matrix and the insertion rod has a significant drawback. When forming a sleeve-shaped article, it is impossible to provide uniform distribution of density along the height of the article as areas of its side surfaces (internal and external) cannot be equal. In compacting hard-to-form powders this causes stratification of long-length articles, leads to nonuniform shrinkage and inadmissible changes in the shape in further sintering.
It should be noted that the constraints currently placed on purity of materials in powder technology, the quest for reduction in costs and simpler preparation operations prohibit the use of lubricants in manufacture of critical parts. Furthermore, reduction of wall friction parameters by process lubricants will prevent meeting the uniform density requirement. The difference in the area relation of the counter moving parts of the passive shaping surface will lead to the necessity to choose a lubricant composition reducing the wall friction factor in proportion to this relation.
A mold is known for uniaxial compacting of powder articles in a sealed volume, that comprises three shaping members: a matrix and two: punches. The punches directly receive the pressing force by a section following the shape of end faces of the article which is formed by active shaping surfaces of the punches, while the matrix defines a side surface of the article, parallel to the pressing axis, and receives lateral pressure force from the compacted powder by its passive shaping surface (Fedorchenko I. M., Frantzevich I. N., Radomyslensky I. D. et al., Powder Metallurgy. Materials, Processing, Properties, Fields of Application, Kiev, Naukova dumka, 1985).
The section of the shaping member of the mold, that receives the pressing force, depends on the hydraulic area of the powder article. This makes requirements to the mold material quality more stringent, reduces service life of the mold and substantially restricts the permissible range of compaction pressures, especially for the articles having a small section in the direction of the pressing axis.
The presence of closed passive shaping surfaces on the mold members, that are hard to reach directly and the quality of which must meet stringent requirements, makes their processing in the manufacturing process and maintenance in operation of the mold more difficult.
When long-length articles of plastic powders that are prone to entrapping air, or powders with a high content of liquid or process lubricant are fabricated in the prior art mold, their removal from the closed volume of the matrix cavity in the compacting process is problematic.
Most closely related to an apparatus for implementing a method in accordance with the present invention is a mold for compacting sleeve-shaped powder articles, comprising a pair of one-piece or composite shaping members that form a shaping cavity defined by active and passive shaping surfaces, the shaping members being arranged so as to mutually move along a pressing axis, with the compaction pressure transferred from the shaping members to the powder material through the active shaping surfaces, while the passive shaping surfaces serve to form surfaces of the powder article that are parallel to the pressing axis (see e.g. Fedorchenko I. M., Frantzevich I. N., Radomyslensky I. D. et al., Powder Metallurgy. Materials, Processing, Properties, Fields of Application, Kiev, Naukova dumka, 1985).
In case of monolith combination of one punch and the insertion rod in one shaping member, and the other punch and the matrix in the other shaping member, the mold allows the permissible range of compaction pressures to be somewhat extended.
However, rigid connection of the matrix and one of the punches substantially hampers or even prevents removal of the compacted article. Furthermore, the apparatus exhibits a structural restraint to increasing the punch section receiving the pressing force due to its combination with the insertion rod. Actual gain in extension of the compaction pressure range is therefore insignificant.
Analytic reasoning of a compacting method in accordance with the invention and derivation of expression for density differential along the height of a powder article produced by the method.
Referring to the drawings, FIGS. 1a, b, c shows molds for uniaxial single-action compacting powder materials into a cylinder article. FIGS. 2a, b, c shows respective schematic diagrams of compacting the powder article. Solid lines show places of mobile mating of parts of a common shaping surface in the compacting process.
FIG. 1a shows a prior art mold for implementing a prior art method of uniaxial single-action compacting of a powder material into a cylinder article, FIGS. 1b, c—molds according to the invention. the mold comprising a first shaping member 1 with an active shaping surface 2, and a second shaping member 3 with a passive shaping surface 4, which define a shaping cavity 5. FIG. 2a shows a schematic diagram of compacting a powder material into a cylinder body.
At the powder body segment adjacent to line 6 (FIG. 1a) of a mobile mating of the active shaping surface 2 of the shaping member 1 and the passive shaping surface 4, i.e. around circumference A′, B′, C′ and D′ (FIG. 2a), the values of powder movement relative to the passive shaping surface, wall friction force and compaction ratio are maximum. At places of fixed mating, i.e. around circumference A, B, C and D, the movement and wall friction force are close to zero and, respectively, the powder compacting ratio is minimum (see e.g. Shtern M. B. et al. Phenomenological Theories of Compacting Powders, Kiev, Naukova dumka, 1982, page 140).
At half the distance between the mobile and fixed active shaping surfaces, i.e. around circumference a, b, c, d, the above values are average. The average density value in section a, b, c, d is the average density throughout the volume of the compacted article. Density distribution in the compacted body along its height is a function of the article height-to-diameter relation and may be uniform only if the compacting ratio is 100%.
From the theory of compacting powder materials in closed molds (see e.g. Popilsky P. Y. et al. Compacting Powder Ceramic Masses, Moscow, Metallurgy, 1983) it is known that due to the wall friction the density differential Δp along the height of the compacted article in its central part (FIGS. 2a, b) along axis EeE′ will be always smaller than that near walls (along line AaA′), while the average density value <ρ> along any vertical will be the same at any instant of compacting:Δρ[AaA′]≧Δρ[EeE′]  (3)<ρ>[AaA′]=<ρ>[EeE′]=<ρ>[CcC′]  (4)
FIG. 1a shows a known mold for compacting powder materials into articles. In a schematic diagram of compacting a cylindrical powder article by a method in accordance with invention (FIG. 2b) in the powder body region adjacent to the line of mobile mating of counter moving parts of the passive shaping surface A′A and C′C, to the left of the line in the region of points A and C, vertical displacement of the powder relative to the passive shaping surface and wall friction forces are close to zero, while to the right of these points the above values are maximum.
In the vicinity of points A′ and C′ the picture is opposite: to the left of the points the above values are maximum, while to the right they are minimum. Therefore, at any points of the powder article being compacted, adjacent to lines A′A and C′C, values of displacement, wall friction forces with account of sign, and the compaction ratio will be equal to the average values between respective values on different sides of the lines of conjugation.
Consequently, the powder compaction ratio in the region along the line of conjugation of parts of the passive shaping surface A′A and C′CX will be the same. According to relations (3) and (4) at any point of the powder article section plane (hatched region A′ACC′) passing through conjugation lines, symmetrical to the central pressing axis, of parts of the passive shaping surface A′A and C′C, the compaction ratio and, hence, the article density will be the same and equal to the average value throughout the article volume.
In the section planes equidistant from the conjugation regions of parts of the passive shaping surface—plane BB′D′D (FIG. 2b), the density distribution must follow classic representations with inversion of the parameters in the region of axis of symmetry E′E of the compacted article, along which the density is constant.
Therefore, at points B′ and D the powder will undergo intense compaction, while at points B and D′ the compaction ratio will be minimum.
Consider the relationship of forces acting in an elementary layer (FIG. 3) with height dh of a powder article compacted by a method in accordance with the invention, wherein the opposite side surfaces of the layer are formed by counter moving parts of a continuous, split along the pressing axis, passive shaping surface of a mold. Assume that values of hydraulic area S0 of the compacted article and total hydraulic perimeter G (which is equal to the sum of perimeters G′ and G″ of counter moving parts), wall friction factors f′ and f″ and lateral pressure ξ are constant along the height; the moment of pair of forces F′fr and F″fr can be neglected; the pressure variation caused by powder mass transfer in the direction normal to the pressing axis is absent.
The force acting on the upper base of the layer having thickness dh is:F0=P·S0  (5)
The force reaction acting on the lower base of the layer is:Fh=(P−dP)·S0  (6)
where dP is the loss of compaction pressure along height dh.
The wall friction force developed at the part of the passive shaping surface that moves in concert with upper active shaping surface is determined by the expression:F′ƒr=Flatƒ′=P ·ξ·S″·ƒ′=P·ξ·ƒ′·G′·dh  (7)
where:                Flat is the lateral stress force;        S′ is the area of the respective part of the passive shaping surface;        G′ is the part of the total hydraulic perimeter, relating to the part of the passive shaping surface having area S′;        f′ is the factor of the wall friction acting on surface S′.        
The wall friction force developed on the part of the passive shaping surface that moves in concert with the lower active shaping surface is:F″ƒr=Flatƒ″=P·ξ·S″·ƒ″=P ·ξ·ƒ″·G″·dh  (8)
In the state of static balance of forcesF0=Fh+″ƒr−F″ƒr,P·S0=(P−dP)·S0+P·ξ·ƒ′·G′·dh−P·ξ·ƒ″·G″·dh  (9)
By integrating, obtain                               ln          ⁡                      (                                          P                h                                            P                0                                      )                          =                              -                          (                                                                    G                    ′                                    ·                                      f                    ′                                                  =                                                      G                    ″                                    ·                                      f                    ″                                                              )                                ·          ξ          ·                      h                          S              0                                                          (        10        )            
The value of density differential along the article height is:                     Δρ        =                  b          ·                      (                                                            G                  ′                                ·                                  f                  ′                                            -                                                G                  ″                                ·                                  f                  ″                                                      )                    ·          ξ          ·                      h                          S              0                                                          (        11        )            
By changing the perimeters to the areas of oppositely directed parts of the passive shaping surface, obtain                               Δ          ⁢                                           ⁢          ρ                =                  b          ·          ξ          ·                                    (                                                                    S                    ′                                    ·                                      f                    ′                                                  -                                                      S                    ″                                    ·                                      f                    ″                                                              )                                      S              0                                                          (        12        )            
Validity of the assumptions made in deriving the equation of density distribution along the article height is determined by the following.
Majority of articles produced by compacting in closed molds have a regular geometric shape without changes in lateral sizes along the pressing axis. In compacting articles having a varying height it is necessary to choose a compacting direction that would satisfy the above requirements to the most extent.
In implementing the method for compacting articles of powder materials in closed rigid molds, the wall friction and lateral pressure factors vary along the article height (see e.g. Popilsky P. Y., Pivinsky Y. E. Compacting Powder Ceramic Masses, Moscow, Metallurgy, 1983). However, numerous experimental data have shown that the product of theses values is constant for the same material being compacted under the same conditions at any compaction pressure.
Furthermore, in the present method variations of these values along the pressing axis are cancelled out since they vary in the same way, but in opposite direction. The moment of a pair of oppositely directed wall friction forces during compacting a powder material leads to bending the layers of the same density in the volume of the article compacted. But since the bend value in the regions adjacent to the parts of the passive shaping surface that move in opposite directions will be the same, with different bend direction in the layers, uniform distribution of the cross-sectional average density value is maintained along the pressing axis during the entire compacting process in the method in accordance with the invention. In addition, the presence of the moment of a pair of oppositely directed forces results in increased plastic deformation ratio of the powder material with the dominating shear component, this promoting the formation of a fine-grain (nanocrystal) structure in fabrication of structural and functional articles, and providing the attainment of the object set.
At the segments adjacent to the conjugation line of the counter moving parts of the passive shaping surface, the powder will be transferred in direction normal to the pressing axis due to the presence of the density gradient on both sides of this line. The powder mass transfer in the volume of the compacted article will lead to a change in the character of distribution of the powder body density. However, if there is a large number of parts of the passive shaping surface that move in different directions relative to the article compacted (FIGS. 1b, 2c), the regions with increased and reduced density will be located side-by-side and change from one region to another through the vertical section regions having the average density (hatched regions).
The closer will be the regions with different compacting character (the more frequently they alternate), the easier the mass transfer occurs between the regions, and the more intense is the shear component of the powder material deformation. In addition, redistribution of the compacted material will occur during the entire compacting process, this equalizing the density throughout the volume since the effective character of its distribution will manifest itself in the compacted article from the instant of minimum load application when the powder is in the bulk density state and its redistribution is not yet restricted by strong bonds, and friction forces between the particles are minimum.
As in any section plane of the powder body normal to the compacting direction according to the present method the reduced and increased density regions alternate with the regions having the average density throughout the article volume, the average density in the sections is the same at any height of the article. Appearance, along the lines of conjugation of parts of the common passive shaping surface split along the pressing axis, of wall friction forces having opposite direction but the same total value leads to equalizing the density throughout the article volume.
In the present method, the density distribution in a powder article is equalized throughout the volume by oppositely directed wall friction forces.